To ensure the quality of printed products produced with a rotary printing machine, it is necessary to check the register accuracy of printed images printed in the rotary printing machine on both sides, i.e. on two opposite sides, of a printing substrate. This check, which is necessary particularly in the production of more expensive printed products, e.g. products to be produced in security printing or high-quality package printing processes, is carried out at least during the makeready phase of the rotary printing machine, i.e. before the rotary printing machine goes into production, i.e. before it begins production printing, wherein during the makeready phase, modules of the rotary printing machine that will be involved in the production process are adjusted in terms of their interaction in such a way that waste paper resulting from poor register accuracy is avoided as much as possible. The term register is understood here as a matching of printed images printed onto the front and back sides of the printing substrate. This means that print images printed onto the two opposite sides of the printing substrate must be in a prescribed relationship in terms of their position relative to one another, preferably factoring in specified tolerances, and this relationship must remain as unchanged as possible during production printing on the rotary printing machine, since any deviation from a setpoint value specified for this relationship, in particular a deviation that exceeds the specified tolerances, will have a negative impact on quality. The relationship involving the relative position of print images printed onto both sides of the printing substrate consists, e.g. in these print images being arranged as congruently as possible, or complementing one another to form a single geometric figure, or e.g. separated from one another in their respective positions at a fixed distance from one another, e.g. defined by the setpoint value. The print images may be embodied, e.g. as any geometric figures or image elements or decorations, or as characters or as symbols or as page numbers or as a full type area. The print image may be a security feature, e.g. of a banknote or of some other security document. This security feature is configured, e.g. such that on the front and back sides, at one point on the banknote or the security document, e.g. partial decorations are located which, when viewed against a light source, i.e. viewing through the document, combine to form a complete decoration. Such security features are preferably produced in what is known as a simultaneous printing process. The printing substrate is preferably embodied as at least one printing sheet and consists, e.g. of paper, cardboard or paperboard.
From DE 102 60 124 A1, a transparent security element for security documents is known, comprising superimposed patterns on two opposite sides of a translucent substrate material, the two patterns complementing one another when arranged true to register to form a complete image.
From DE 10 2007 053 594 A1, a device for determining perfecting register in the case of a printing sheet printed on both sides is known, in which a first register element on the recto side and a second register element on the verso side of the printing sheet can be illuminated, and the relative positions of the illuminated register elements can be compared, said device comprising a beam deflecting device which has an intake for insertion of a printed sheet bearing the register elements and which mirrors the images reflected by the illuminated register elements one above the other or side by side, allowing the position of the register elements to be assessed from a display monitor.
From DE 199 45 979 C1, a device for measuring measurement marks on the upper side and the underside of a printed product relative to one another and relative to a printed product edge is known, said device consisting of two measurement transducers, each associated with one side of the printing substrate, at least one reference mark carrier, and a clamping device, wherein the clamping device consists of two clamping elements, the clamping elements being mounted rotatably relative to one another about a common bearing axis of a bearing and each of the clamping elements extending in the bearing axis having a longitudinal slit which together form a clamping gap when closed, wherein the printed product can be inserted into this clamping gap and secured by rotation of the clamping elements, and the measurement marks including the printed product edge lie in the region of the slit opening.
From DE 298 07 663 U1, a device for determining the perfecting register of sheets printed on the rector and verso sides is known, wherein a measuring device assigned to the recto side of the sheet and a measuring device assigned to the verso side of the sheet are provided for detecting the position of points on the recto and verso sides of the sheet, and the sheet can be fed in between the two measuring devices and fixed in a position designated for the measurement.
From DE 298 19 735 U1, a device for determining the perfecting register of a sheet printed on both sides is known, in which a first register element on the recto side of the sheet and a second register element on the verso side of the sheet can be illuminated, and the positions of the illuminated register elements relative to one another can be compared, wherein a beam deflecting device that includes a sheet intake for insertion of a sheet section bearing the register elements mirrors the images reflected by the illuminated register elements one above the other or side by side in such a way that the position of the register element on the recto side and the position of the register element on the verso side can be assessed relative to one another on a display monitor.
From JP H05 254 105 A, a register mark inspection device is known, which inspects the positional relationship between register marks on the front side and the reverse side of a printed product, wherein the register mark inspection device has two lenses, each of which generates an image of one of the register marks on the front side and the reverse side of the printed product, and which are arranged opposite one another.
From DE 103 22 547 A1, a method is known for determining and correcting register deviations on sheets that are printed on both sides in sheet-fed printing machines that include a measuring device for sheet inspection, which has a measuring table and a measuring head that is movable parallel to the measuring table, wherein the measuring table is equipped with vertical and horizontal sheet stops outside of the maximum sheet format, and the positioning system of the movable measuring head is capable of detecting the position of measuring points on the sheets to within 1/100 mm, said method comprising the following steps:                printing register marks onto the front and reverse sides of a sheet during perfecting printing in a sheet-fed printing machine,        placing the sheet on the measuring table and aligning the sheet against horizontal and vertical sheet stops associated with the same corner of the sheet,        determining the horizontal and vertical distance of the register mark associated with said corner of the sheet on the front side of the sheet from the nearest lateral edges of the sheet using a positioning system in the measuring head,        turning the printing substrate sheet and aligning the same corner of the sheet against the opposite stops in the direction of turning,        determining the horizontal and vertical distance of the register mark associated with the same corner of the sheet on the back side of the sheet from the nearest lateral edges of the sheet using the positioning system in the measuring head,        calculating the differences in position between the register marks on the front and reverse sides of the sheet,        correcting the position of the printing cylinders using the determined position differences as a basis, to produce congruence of the register marks.        
From EP 2 447 071 A1 a lateral register correcting device is known, having a register correcting unit, which is capable of correcting register movement of a fiber web through each of a plurality of printing units, and a fan out correcting unit, which is capable of correcting a width of the fiber web stretched in a widthwise direction, the lateral register correcting device comprising: a) a register detection and estimation unit capable of estimating and detecting a register correction amount by the register correcting unit based on at least two register marks, which are printed onto the fiber web by each of the printing units when a plurality of printing units are printing on an advancing fiber web for carrying out a correction by means of the register correcting unit and the fan out correcting unit; b) a register correction control unit capable of automatically controlling the register correcting unit in real time based on the register correction amount derived by the register detection and estimation unit; c) a fan out detection and estimation unit capable of estimating and detecting a fan out correction amount by the fan out correcting unit based on a register mark; and d) a fan out correction control unit capable of automatically controlling the fan out correcting unit in real time based on the fan out correction amount derived by the fan out detection and estimation unit, wherein the register marks are printed onto a front side and a reverse side of the fiber web.
From U.S. Pat. No. 5,036,763, a convertible multi-color printing machine for recto and verso printing, in particular of banknotes, is known, said machine being composed of two substantially identically constructed halves, each having a first cylinder, an image transfer cylinder pressed against this first cylinder, a second cylinder embodied as a blanket cylinder, which is displaceable between an offset printing position, in which it is pressed against the first cylinder, and a collect printing position, in which it is moved away from the first cylinder, a plurality of convertible cylinders in contact with said blanket cylinder, which are formed as offset plate cylinders in the offset printing position and as ink selection cylinders in the collect printing position and can each be inked up by an inking unit, a displaceable collect printing plate cylinder, assigned to the blanket cylinder, which in the offset printing position can be moved away from the blanket cylinder to a non-operating position and which in the collect operating position cooperates both with the blanket cylinder and with the image transfer cylinder, and in at least one of the machine halves, an additional printing unit having a plate cylinder inked up by a dedicated inking unit, wherein the first two cylinders of each machine half function as adjacent printing cylinders lying side by side, wherein a first printing nip is formed between the printing cylinder and the image transfer cylinder of each machine half, and in the offset printing position, a second printing nip is formed between the printing and blanket cylinders pressed against one another, wherein the paper to be printed first wraps partially around one printing cylinder in one direction, and after passing through the nip between the two printing cylinders wraps partially around the other printing cylinder in the other direction, wherein the paper passes through the printing nips in both machine halves during printing first of the one side of the paper and then the other side of the paper, and wherein the plate cylinder of the additional printing unit inks up the image transfer cylinder.